(1) Field of the Invention
The invention relates to a centrifugal blower; in particular to a centrifugal blower improved to reduce the vibration and noise thereof.
(2) Description of the Related Art
Some centrifugal blowers comprise a motor, a centrifugal fan which is driven by the motor, and a casing, for enclosing the centrifugal fan, which defines a duct in the form of a spiral shape. The shaft of the motor is disposed within the casing and connected to the centrifugal fan.
When the area of the opening of the duct is reduced from fully opened position to fully closed position with the motor rotating constantly, the pressure in the duct changes with relation to the flow rate. In FIG. 14, the change of pressure relative to the flow rate in a centrifugal blower is illustrated. First, the pressure in the duct increases with the increasing flow rate. The increase is the pressure will be saturated at a level of the flow rate, above which the pressure decreases. The surging range is defined as the range in which the pressure increases with the increasing flow rate. In the surging range, pulsation of the pressure, so called "surging", is generated which makes the operation of the blower unstable. In the surging range, the efficiency of the blower is reduced and the vibration and noise in the blower is increased.
An air conditioner for an automobile uses a centrifugal blower which must operate at various flow conditions to adjust the desired flow rate, even in the surge region.